1. Field of the Invention
The present invention relates to a tuner circuit and a digital broadcast receiver. In particular, the present invention relates to a tuner circuit and a digital broadcast receiver each receiving a television signal of a digital broadcast.
2. Description of the Background Art
About 50 years have elapsed since an analog television broadcast started. At present, a digital terrestrial television broadcast and a digital cable television broadcast tend to be mainstream. A digital broadcast is superior to an analog broadcast in terms of image quality, the number of channels, and functions. Hence, there is an increasing demand for a digital broadcast receiver such as a TV receiver, a VTR (Video Tape Recorder) or a STB (Set Top Box) capable of receiving a digital broadcast.
FIG. 9 is a block diagram illustrating a schematic configuration of a conventional digital broadcast receiver 200. As illustrated in FIG. 9, a tuner circuit 101 includes a wide band amplifier 111, an RF (Radio Frequency) input filter 112, an RF-AGC (Automatic Gain Control) amplifier 113, an RF interstage filter 114, a PLL (Phase Locked Loop) circuit 115, a local oscillator 116, a mixing circuit 117, an IF (Intermediate Frequency) amplifier 118, an automatic gain control circuit 119, an emitter follower circuit 120, an If filter 121, an IF-AGC amplifier 122, an A/D (Analog/Digital) converter 123, a digital demodulation circuit 124, and an error correction circuit 125.
Wide band amplifier 111 receives a television signal (RF signal) received by an antenna 102, amplifies the signal over a wide frequency band, and outputs a resultant signal. RF input filter 112 receives the signal output from wide band amplifier 111, removes an unnecessary component from the received signal, and allows only a signal in a desired frequency band to pass therethrough. RF-AGC amplifier 113 has a variable gain, radio frequency amplifies the signal (RF signal) passing through RF input filter 112, and outputs a resultant signal. RF interstage filter 114 receives the signal output from RF-AGC amplifier 113, removes an unnecessary component from the signal, and allows only a signal in a desired frequency band to pass therethrough.
Each of PLL circuit 115 and local oscillator 116 generates a local oscillation signal at an oscillation frequency corresponding to a frequency of a desired channel. Mixing circuit 117 mixes the signal output from RF interstage filter 114 with the local oscillation signal output from local oscillator 116, and frequency converts a resultant signal to an IF signal. IF amplifier 118 amplifies the IF signal output from mixing circuit 117, and outputs a resultant signal. Automatic gain control circuit 119 automatically controls the gain of RF-AGC amplifier 113 in accordance with a level of the signal output from IF amplifier 118.
Emitter follower circuit 120 is an impedance conversion circuit having a high input impedance and a low output impedance, and current amplifies the signal output from IF amplifier 118. IF filter 121 is, for example, a SAW (Surface Acoustic Wave) filter, receives the signal output from IF amplifier 118 through emitter follower circuit 120, removes an unnecessary component from the signal, and allows only a signal in a desired frequency band to pass therethrough. IF-AGC amplifier 122 has a variable gain, amplifies the signal (IF signal) passing through IF filter 121, and outputs a resultant signal. Herein, emitter follower circuit 120 plays a role of suppressing generation of a distortion signal in tuner circuit 101. Therefore, emitter follower circuit 120 is inserted so as to prevent distortion performance of tuner circuit 101 from being degraded even when IF filter 121 provided at a poststage has a low impedance.
A/D converter 123 converts the signal output from IF-AGC amplifier 122 from an analog form to a digital form. Digital demodulation circuit 124 receives the digital signal output from A/D converter 123, and performs digital demodulation such as QAM demodulation or OFDM demodulation on the digital signal. Further, digital demodulation circuit 124 automatically controls the gain of IF-AGC amplifier 122 in accordance with a level of the signal output from A/D converter 123.
Error correction circuit 125 receives the signal subjected to digital demodulation by digital demodulation circuit 124, corrects an error generated due to an influence of noise and the like, and generates a TS (Transport Stream) signal. A/D converter 123, digital demodulation circuit 124 and error correction circuit 125 form a demodulation IC (Integrated Circuit) 126 integrated into one chip. The TS signal output from error correction circuit 125 is converted to a video signal, an audio signal and a data signal by a signal processing circuit 103 in digital broadcast receiver 200; thus, these signals can be displayed on a monitor 104 in an audio-visual manner.
FIG. 10 is a circuit diagram illustrating a configuration of emitter follower circuit 120 illustrated in FIG. 9. As illustrated in FIG. 10, emitter follower circuit 120 includes a transistor 131 for current amplification (e.g., bipolar transistor), and resistor elements 132, 133 and 134. Transistor 131 has a base B connected to an output node of IF amplifier 118 illustrated in FIG. 9, a collector C connected to a line of a power supply potential Vcc, and an emitter E connected to an input node of IF filter 121 illustrated in FIG. 9. Resistor element 132 is connected between collector C and base B in transistor 131. Resistor element 133 is connected between base B of transistor 131 and a line of a ground potential GND. Resistor element 134 is connected between emitter E of transistor 131 and a line of a ground potential GND.
In emitter follower circuit 120, each of resistor elements 132 and 133 on an input side has a large resistance value, and resistor element 134 on an output side has a small resistance value. Hence, emitter follower circuit 120 acts as an impedance conversion circuit having a high input impedance and a low output impedance. Resistor element 134 on the output side has a small resistance value; therefore, emitter follower circuit 120 can feed a relatively large current to a load connected to an output node and can withstand a load with a small resistance value.
With reference to FIG. 9 again, next, description will be given of an automatic gain control operation of each of RF-AGC amplifier 113 and IF-AGC amplifier 122. As illustrated in FIG. 9, in general, a digital broadcast receiver performs automatic gain control for an RF signal system and, also, performs automatic gain control for an IF signal system.
Automatic gain control circuit 119 detects a level of the signal output from IF amplifier 118, and generates an RF-AGC voltage for automatically controlling the gain of RF-AGC amplifier 113 such that the signal output from RF-AGC amplifier 113 is kept at a certain level, in accordance with the detected signal level. Specifically, if the signal output from IF amplifier 118 has a low level, automatic gain control circuit 119 increases the gain of RF-AGC amplifier 113 to thereby prevent degradation of a noise factor (NF). On the other hand, if the signal output from IF amplifier 118 has a high level, automatic gain control circuit 119 decreases the gain of RF-AGC amplifier 113 to thereby prevent degradation of distortion performance of tuner circuit 101. As described above, automatic gain control for an RF signal system is performed by detection of the level of the signal output from IF amplifier 118 in many cases.
Digital demodulation circuit 124 generates an IF-AGC voltage for automatically controlling the gain of IF-AGC amplifier 122 such that the signal output from IF-AGC amplifier 122 is kept at a certain level, in accordance with a level of the signal output from A/D converter 123. Specifically, if the signal output from A/D converter 123 has a low level, digital demodulation circuit 124 increases the gain of IF-AGC amplifier 122 to thereby optimize demodulation performance of demodulation IC 126. On the other hand, if the signal output from A/D converter 123 has a high level, digital demodulation circuit 124 decreases the gain of IF-AGC amplifier 122 to thereby optimize the demodulation performance of demodulation IC 126. As described above, digital demodulation circuit 124 has a function of performing automatic gain control for an IF signal system in many cases.
Herein, wide band amplifier 111 is provided for improving reception sensitivity of tuner circuit 101 over a wide frequency band. However, if the reception sensitivity is improved, but the signal received by antenna 102 has a strong input level (not less than 90 dBuV), emitter follower circuit 120 generates a distortion signal in some cases.
FIG. 11 shows distortion performance of emitter follower circuit 120. With reference to FIG. 11, if antenna 102 receives no interference signal, emitter follower circuit 120 generates no distortion signal. In a frequent case, even when a received interference signal has a level almost equal to that of a television signal, a distortion signal generated by emitter follower circuit 120 causes no problem.
However, in some cases, for example, an antenna station transmitting a television signal is located at a faraway place and an antenna station transmitting an interference signal is located nearby and, alternatively, a television signal has a considerably low level because radio waves are cut off by buildings and the like. Under the aforementioned peculiar conditions, as shown in FIG. 11, there is a possibility that a level of an interference signal is higher than that of a television signal by not less than 40 dB. In such a case, emitter follower circuit 120 generates a distortion signal, so that an influence of spurious interference is not negligible.
In view of this problem, there is demanded realization of a high-performance digital broadcast receiver capable of favorably receiving a desired television signal without an influence of an interference signal even under the aforementioned peculiar conditions.
Japanese Patent Laying-Open No. 06-153100 discloses a method of preventing saturation of radio frequency amplification when a reception electric field between adjoining channels is large and preventing interference between the adjoining channels based on this saturation, in a TV tuner device.
As described above, there is demanded realization of a high-performance digital broadcast receiver capable of favorably receiving a desired television signal without an influence of an interference signal even when a level of an interference signal is higher than that of a television signal by not less than 40 dB.
In order to improve the distortion performance of emitter follower circuit 120, it is effective that a drive current of transistor 131 included in emitter follower circuit 120 is made large. However, a demand for power saving is increased in recent years. Therefore, even when the drive current of emitter follower circuit 120 is simply made large, power consumption is undesirably increased. In addition, heat generation due to a large drive current causes a problem. As for mobile application, a battery has a limited lifetime; therefore, a large drive current is undesirable.